Plastic electronics, where functional devices are fabricated on, or are transferred to polymer substrates has been proposed as the future for large area systems such as displays, detector arrays and transducer arrays. Particularly when used to fabricate functional electronics for display applications, whether this is for Organic Light Emitting Diode (OLED), Liquid Crystal Displays (LCD), or Electronic Paper Displays (EPD), the number of layers to be fabricated or transferred to the polymer substrate is large, as is the number of functional processing steps that must be applied.
Typically, processes involving large numbers of layers to be fabricated over large areas with a large number of functional processing steps, such as when processing substrates for displays, a number of issues may be encountered.
These issues include ensuring good realignment of micron scale patterning, i.e. ensuring good alignment between different layers and features laid down in different processing steps. For example, when fabricating a simple three terminal TFT device, it is critical that the gate is correctly aligned to the source/drain contacts. For a polymer substrate dimensional stability is often poor, and measured in percent, as opposed to parts per million (ppm) for more traditional substrate materials such as glass or silicon, and therefore the processes used in the fabrication of devices on a polymer substrate typically relies on some level of self-alignment, serial processing (such as inkjet or aerosol deposition) or on slack design rules to ensure operation even with poor alignment.
The film surfaces used may include defects, even when high quality polymer substrates specifically produced for electronics use are used. Typically these defects include particulate, scratch or inclusion (bubble) types, and they can be a significant source of failure of devices and interconnects fabricated on the polymer substrate, leading to a very poor areal yield for the fabricated devices. This is critical when fabricating large area devices such as display and sensor arrays. These defects may be present in the original fabrication of the film, but can also be generated during the handling and processing, particularly during vacuum deposition of metals and polymers.
Furthermore, in order to identify and monitor yield problems during the fabrication process, high quality inspection is required to avoid processing substrates including defects, rendering any fabricated devices non-operative, through multiple, potentially expensive, further processing steps.